Semiconductor lasers are commonly used in modern technology as a light source in various devices, including communication devices, such as fiber optic transmitters, and compact disc players. A typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap low refractive index layers. The low bandgap layer is termed the “active layer” and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types, and when current is passed through the structure, electrons and holes recombine in the active layer to generate light.
Surface-emitting, rather than edge-emitting lasers have been developed. One surface-emitting laser is a “vertical cavity surface emitting laser” (VCSEL). Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry.
Surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices cannot be as easily fabricated. An array of lasers can be fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately connected with appropriate contacts. Such arrays are potentially useful in such diverse applications as, for example, image processing, inter-chip communications (i.e., optical interconnects), and so forth. Typical edge-emitter lasers are turned on and off by varying the current flow through the device. This often requires a relatively large change in the current through the device which is undesirable. In comparison, surface-emitting lasers often require lower drive current, and thus the change of current to switch the VCSEL need not be as large.
High-yield, high performance VCSELs have been demonstrated, and exploited in commercialization. Surface-emitting AlGaAs-based VCSELs are producible in a manner similar to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) chamber giving very high device yields.
VCSELs typically have an active region with bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the high-reflectance mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is typically turned on and off by varying the current through the active region.
VCSELs may have multiple aluminum bearing Group III-V layers. In particular, the VCSEL may have AlAs layers and AlGaAs layers. The aluminum bearing layers are protected from the environment in a vertical direction by the top surface, which can include one or more surface passivation layers. The aluminum bearing layers typically are exposed to the environment at the edges or side face surfaces, particularly after the wafer has been cut into individual die. The aluminum bearing edges can oxidize when the chip is placed into service in an oxidizing environment. The environmentally induced oxidation can cause unreliable oxidation, from the edge inward, of the aluminum bearing layers. If left unchecked, this lateral oxidization can sometimes reach the VCSEL device itself, thereby reducing performance or even preventing operation altogether. To prevent or inhibit such lateral oxidation of the aluminum bearing layers, the chip is commonly mounted in a hermetically sealed package. Hermetically sealed packages have a number of limitations. First, hermetically sealed packages can be relatively expensive, which increases the cost of the device. Second, hermetically sealed packages can be relatively bulky, which increases the space needed to mount the device on a circuit board or multi-chip package, both of which are undesirable.
What would be desirable is a method for sealing the edges of chips, particularly VCSEL chips, prior to cutting the chips from the wafer. VCSEL chips having sealed edges may not require a hermetically sealed package.